Magnetic field responsive device



June 28, 1949. R. G. RowE 2,474,693 MAGNETIC FIELD `RIISPCHSIVEI DEVICE Filed aan. 51, 1945 5 sheets-sheet 1 g l i e t 1. E

. i a i i a g c" l V l flH June 2s, 1949. R. G. o'wE 2,474,693

MAGNETIC FIELD RESPQNSIVE DEVICE Filed Jan. 3l. 1945 3 Sheets-Sheet 24 June 28, 1949. R. G. RowE 2,474,693

MAGNETIC FIELD RESPONSIVE DEVICE Filed Jan. 51. 1945 3 Shee'ts-Sheet 3 Hy@ N Patented June 28, 1949 MAGNETIC Firm nEsPoNslvs nevica Robert G. Rowe, Niagara Falls, N. Y. .q

Application January 31, 1945, Serial No. 575,438

(Ci. F15-183) 17 Claims.

This invention relates, in general, to magnetic held responsive devices and, in particular, to4 a novel and useful method and apparatus for indicating the magnitude and/or direction of magnetic iields or the deviation from the earths magnetic meridian.

While the present invention is diierent in principle and character from other magnetic field responsive devices, it may be most closely compared with earth inductor compasses of atype known generally as magneto or tiux gate compasses. Certain limited parallels of lfunction can similarly be drawn'between the present invention and United States Patent 4Number 2,- 331,617, issued on October 12, 1943, to David W. Moore,` Jr.

Prior art Compasses under the general classification of earth inductor compasses and, `in particular, magneto compasses, require an oscillating or rotating membermounted in pivots or bearings and have thedisadvantages of critical bearing tension, bearing wear, bearing adjustment and other faults resulting from frictional phenomena. In addition, magneto compasses have the further disadvantage of requiring an auxiliary driving motor device coupled to the generator being excited by the earth's held. These compasses suffer the disadvantages of high expense and complexity inherent in mechanical systems having numerous moving parts operating at high rotational speeds. Further, they require a means for commutation which introduces dif ilculties arising from commutator and brush wear. Still further, they produce low power outputs which are, in manypases, nsulcient for control or for driving r/epeater compass indicators.

The magnetic field responsive device disclosed in United States Patent Number 2,331,617, while overcoming some of--the disadvantages of prior art earth inductor compasses, still retains the disadvantage of requiring a, delicate pivotally mounted oscillating member. In addition, a further disadvantage occurs due to the care which must be exercised in balancing the oscillating lmember with respect to the alternating current exciting winding.

Therefore, an object of the present invention is to provide a magnetic field responsive device, suitable for use in the earth inductor compass. which eliminatespivotally mounted oscillating parts. l

Another object of the present invention is to provide a, magnetic field responsive device, suitable for use in the earth inductor compass, which eliminates rotating parts.

Still another object of the present ,invention is to provide a magnetic eld responsive device. suitable for'use in the earth inductor compass,

with which appreciable amounts of power may be produced.

A further object of the present invention is to provide a novel and improved magnetic eld responsive device.

A still further object of the present invention is Ito provide a magnetic field responsive device which substantially overcomes one or more of the disadvantages of prior art earth inductor compasses.

The foregoing objects and other objects ancillary thereto I prefer to accomplish in short by a novel combination of the magneto-mechanical properties of ferromagnetic or magnetostrictive materials with electric detecting and/or indicating apparatus.

The novel features which I believe to be characteristic of my invention are engendered with particularity in the appended claims; the invention itself, however, will be best understood by reference to the accompanying detailed description and drawings, in which Figure 1 illustrates graphically an' approximate curve of the manner in which a magnetostrictive material changes length in amagnetic iield.

Figure 2 illustrates graphically an expanded portion of the lower knee of the foregoing curve.

Figure 3 illustrates graphically an expanded portion of the upper knee of the foregoing curve.

Figure 4 illustrates diagrammatically and schematically one embodiment of the present invention.

Figure 5 illustrates diagrammatically and schematically another embodiment of the present in- Ventill.

general, magnetostriction refers to that set of i phenomena in which certain materials exhibit a change in dimension upon magnetization.

With reference now to Figure 1, which illustrates an approximate curve a-b-c-d-ef-f-a-h of the manner in which magnetostrictivef material changes length in a magnetic field, H linearly represents the magnetic field intensity, l the original length of the material and lm the modified length of the materialdue to field H. Hence linearly represents the change in length per unit length of' material in field H. It will be appreciated that lm may be either greater or smaller lthan l, depending upon the nature of the material and, for some materials, upon 4the field intensity.

y Nickel only shortens upon magnetization; cobalt shortens for low field intensities and lengthens for high; iron lengthens for low field intensities and shortens for high: certain manganese alloys only lengthen upon magnetization.

With reference to Figures 2 and 3, which illustrate expanded portions of the curve in Figure 1, the primed letters refer to H values and the double primed letters refer to values for the correspondingly lettered points on thefcurve Inspection of Figures 1, 2 and 3 reveals that the slope of the magnetostriction curve,

is not a constant. For example, the change in length attendant upon a unit increase in H from point a.' to point b is smaller than that produced byV unit increase in H from point c' to point d'. Similarly, the change in length attendant upon unit increase in H from point e' to point j' is greater than that produced by `runit increase in H from point g' to point h. Thus, for unit increase in H, a"b is smaller than bc"; b"c" is smaller than c"d"; and so on. The same is` true for decreasing H values beginning at point d on the curve. A similar relationship can be worked out Vfor the upper knee of the curve as shown in Figure 3. Y

From the above disclosure it will be recognized that if a rod of ferromagnetic or magnetostrictive material where suspended in the field of the earth, for example, the length' of -the rod would change in accordance with its position in the field. If the longitudinal axis of the rod were perpendicular to the earths field, thatis in an east-west directionH along the axis of the rod would be substantially zero and hence no change in length would occur. Displacing the longitudinal axis of the rod in a manner such that it becomes parallel Ito the earths field, that is in a north-south direction, H along the axis of the-rod would become greater than zero and a changel in length would /obtain. The change in length attending magnetization by weak fields such Ias that of the earth is so' exceedingly minute, however, that rugged mechanical means'for indication of dimensional modification would be difficult to design and construct. Vibration, thermal eilects and other natixig magnetic field in addition to the steady lfield underobservation, and thatwhenlmatchthef periodicity of the alternating field with some harmonic of a natural elastic longitudinal period oi the rod, I am able to indicate changes in the direction or magnitude of the steady field under observation by measuring the change in the resonant amplitude of vibration of the rod.

For example, with reference to Figure 2, assume that a rod of ferromagnetic or magnetostrictive material has been suiliciently magnetized to give an H value of b. An alternating magnetic field can be applied in addition such that the total value for H will fluctuate around b from b to c', from c' back through b to a', from a' to 4b and so on, at the frequency of the A. C. fleld.` Operation will be over a portion of the curve where the change in length per unit length attending these combinations of fields is relatively small and might be represented by a change from a" to c.

Now assume that an additional steady field which aids the pre-magnetized condition of the rod is added such that H has a new value corresponding to c'. An alternating magnetic field can be added such that the total value for H will fluctuate around c' from c' to d', from d' back through c to b', from b' to c' and so on, at the frequency of the A. C. field. Operation will be over a portion of the curve where the change in length per unit length attending this new combination of fields is greater than in the previous case and might be represented by a change from bl' ,to dll' Whenl the frequency of the applied alternating magnetic field is closely matched to that of a natural elastic longitudinal period of the rod, resonance occurs between the electrical and mechanical systems and the amplitude of rod dis- 40 placement becomes large enough to detect elecdiillculties make mechanical methods impracticalv trically. Setting the center operating'point by pre-magnetization at b, on the curve in Figure 2, produces a certain vibrational amplitude in the rod; whereas setting the center operating point at c, on the curve in Figure 2, produces a greater vibrational amplitude.

I have found that steady magnetic fields of low intensity, such as the earths field, are of suilicient magnitude to shift the operating point on the curve and that I can detect this shift' by electrical and/or electronic means due to the resulting change in the vibrational amplitude of the rod. In Figures 1 and 3, curve e-f-g-h has been included to illustrate the possibility of operation at a different center point on the large curve. For improved indicator linearity or sensitivity it may be desirable to operate over this portion. It is to be understood that all ferromagnetic or magnetostrictive materials do not exhibit the same magnetostrictive curve and that it remains to select that material best suited to desired' 0perating conditions. y

It is to be understood, further, that I do not wish to be limited to the detection of va'riations in vibrational amplitude alone for an indication of the direction or magnitude of the external field. Laterl show and describe methods and apparatus for the detection of phase angle shift. waveform distortion and other pertinent factors.

For a complete understanding of the nature of the present invention as opposed to certain other inventions, it is essential to comprehend the difference between introducing vibrations in a magneto-mechanical system by magnetostriction as opposed to introducing vibrations in -a magnetormechanical system'sy magnetic attraction. In magnetostriction, the periodic ileld causes periodio changes in the dimension ofthe magnetostrictive element duel to the. periodic internal stress attending Aperiodic magnetization by the field coil. In magnetic attraction, the periodic field causes changes in position of the magnetic armature element due to the periodic attractionY between the armature and the field coil. In mas--v netostriction, the field coil may be conveniently wound directly on the magnetostrictive element;

-whereas in magnetic attraction, lthe field coil C must be accurately spaced from the magnetic armature. i

With reference now to Figure 4, magnetostrictive rod I is firmly clamped at its midpoint by' supports 2 and 3. Exciting winding I, which may be wound directly on the rod, is coupled through leads 5 and 6 to a source of alternating current of proper frequency. The frequency of the source oi' alternating current matches the natural mechanical longitudinal period oi?` vibration of the `rod at either its fundamental for some other harmonic period. in one embodiment of the invention. In this modification of the invention rod I may be made of annealed steel approximately ten inches long by one-half inch in diameter, and driven in resonance at its lowest natural longitudinal period by an A. C. source of approximately 10 kilocycles per second. The natural period 6v I am able to indicate the direction and relative intensity of magnetic fields with respect to the longitudinalaxls of rod I.

When rod I is so oriented with respect to a magnetic field, for example the earths neld, in a manner such that the external field is aiding the pre-magnetized condition of rod I. the amplitude.

of longitudinal-vibration in the rod will increase and meter I1. will deflect from its balanced position dueA to the increased voltage output from transducer I0, amplifier Il and rectifier I6. Conversely, when the external magnetic field is bucking the pre-magnetized condition of rod I. the

amplitude of longitudinal vibration in the rod. will decrease vand meter I1 will deflect in a diierthe direction of the vessel with respect to the earthks magnetic meridian may be determined. Further, a mounting may be provided such that the rod is always substantially exposed to only the horizontal component of the earths magnetic field, for installations inairships, or the like, havof such a rod may be represented by the equation where F equals the frequency in cycles per second, l equals the length in centimeters, E equals the modulus of elasticity in dynes per square centimeter and d equals the density of the rod material in grams per cubic centimeter.

l In Figure 4, battery 1, rheostat 8 and winding 9, which may be wound directly on the rod as well,

cooperate to apply slight pre-magnetization to rod I. In practice I have found that this premagnetization may be that of a permanent nature and elements 1, 8 and B may be dispensed with if desirable. Piezoelectric transducer I 0,

which is mechanically connected to rod I, may be ate other frequencies.- Such amplifiers are well` known in the electronic art. Amplifier output terminals I4 and I5 are coupled through rectifier I5 and potentiometer 20 to indicating meter I1.

Resistor I8, battery I9 and potentiometer 2U cooperate to supply a bucking voltage to meter I1 to buck out the initial meter deflection caused by the alternating current output of amplifier I3. Rod I, being slightly pre-magnetized by battery 1, rheostat Il and coil 9, is set into resonant longitudinal vibration by an alternating excitation voltage applied across terminals 5 and 6. 'Ihe mechanical output of the rod is converted into an alternating voltage by transducer III and this A. C. voltage is further amplified by amplifier I3. The output of amplifier I3 is rectified and applied through a balancing circuit to meter II. The combination of resistor I8, battery I9 and potentiometer 20 is so proportioned that the initial defiection of meter I1 can be balanced to zero if desired.

With the apparatus disclosed, I have found that ing three axes `of gyration. Mountings may be pendulously or fgyro stabilized, if desired. Such mountings are bld in the art of earth inductor compasses.`

I' have found, further, that the phase of the voltage `produced by the longitudinal vibration of rod I can be caused to shift with respect to the phase of the exciting voltage, according to the condition of pre-magnetization, the frequency relationship between the driving source and the natural elastic period of the rod and the influence i of weak external fields. With `reference to Figure 10, the correspondingly numbered parts are explained in connection with Figure 4. In Figure 10, the exciting voltage appearing across terminals 5 and 6 is applied to input terminals 2| and 22 of amplitude limiter 23, as well as to coil t of rod I. Output terminals 24 and 25 of limiter 23 are coupled to one pair of phase meter input terminals 2B and ,21. The voltage output of amplifier I3 is similarlyfed to input terminals 28 and 29 of amplitude .limiter 30, and the limiter output, in turn, is fed from terminals 3| and 32 to the second pair of phase meter input terminals 33 and 34.

Amplitude limiters 23 and 30 are well known in the electronic art and are employed in this connection to prevent changes in voltage amplitude from effecting the phase meter. Phase meter 35 is likewise well known in the electronic art and includes a meter and suitable circuit for indicating the diieren'ce in phase or the change in phase between two input voltages of equal frequency and constant amplitude. In place of the apparatus indicated, other phase responsive devices may be suitably employed. In this modification, the frequency of the driving source closely voltage. With rod I parallel to the lines of -f'orce indicate' the direction or magnitude of the external field. s l

Figure illustrates a further modification vof the invention employing a 'type of bridge circuit in which two oppsitely preemagnetized magnetostriction rods are incorporated. Rods 36 and` 31 are nodaily clamped for a preferred modefof longitudinal vibration by supports 38, 39, 40 and 4I respectively. Thesesupports 'are further mechanically connected together to keep the two magnetostrictive rods parallel and in the same plane at all times. In this particular embodimerit of the invention, rods 36 and 31 are out to accurately equivalent dimensions in order 'that both will exhibit elastic resonance at one and the same frequency. `Exciting windings 42-an'd 43 are coupled through leads 44 and 45 to a source of alternating current of the proper frequency. In this modicationof the invention, the exciting frequency may be equal to the natural fundamental longitudinal period of the rods.

Battery 46, rheostat 41 and windings 48 and.

43 cooperate to oppositely pre-magnetize rods 36 and 31 as indicated. Piezoelectrictransducers 50 and 5|, which are mechanically connected to rods 35 and 31, have their outputs connected through amplifiersl 52 and 53 to half-wave vacuum tube rectifiers 54 and 55. Rectifiers 54,and 55', lter condensers 55 and 51 and potentiometer 58 cooperate to supply a D. C. voltage for indicating meter 53. s

Rods 35 and 31, being oppositely pre-magnetized. are set into resonant longitudinal vibration by an alternating excitation voltage applied acrossterminals 44 'and 45. The mechanical ou*- put of the rods ls converted into an alternating current by transducers ,50 and 5| and passed through amplifiers .52 and 53 respectively. The

output of each amplifier is separately rectied 'by n rectifiers 54and 55, filtered by condensers 56 and 51 and applied across potentiometer 58. Withthe longitudinal axes of'rods 36 and 31 perpendicular to an external magnetic field, a point can be found by varying the sliding contact on potentiometer 58 at whichno voltage will appear across meter 59.

With rods 36 and 31 so oriented with respect to an external magnetic field such that the field` is aiding the pre-magnetized condition of rod 35 and simultaneously bucking that of rod 31, tlhe amplitude of vibration of rod 36 will increase and that of rod 31 will decrease. The voltage output of transducer 50, amplifier 52 and rectifier 54 will increase, whereas the voltage output of transducer 5I, amplifier 53 and rectifier 55 will def crease. This difference in voltage appearing across the previously balanced ratio arms of the bridge will cause meter 59 to deflect, for example to the right.

Conversely, with rods 36 and 31 so oriented with respect to the external magnetic field that the field is bucking the pre-magnetized' condition of. y

rod 36`and simultaneously aiding that of rod 31, the amplitude of rod 36 will decrease and that of rod 31 will increase. I'he voltage output of transducer 50, amplifier 52 and rectifier 54 will decrease, whereas the voltage output of -transducer 5|, amplifier 53 and rectifier 55 will increase. This reverse difference in voltage appearing across the previously balanced ratio arms oi the bridge will `cause meter 59 to deflect in a reverse direction, or to the left.

Thus, -whenthe longitudinal axes of rods 36 and 31 are parallel to the flux lines of the external 8 l magnetic field, meter 59 will give a. maxim deflection either to right or left according to the polarity of the externalffleld. When rodsy35 and 31 have their longitudinal axes perpendicular to 5 the flux lines of the external magnetic field.

meter 53,v will not deflect from its normal center position. It is to be understood that other suit- 'L V'ableforms of metering may be employed. 'Ivhe balanced or. bridge type of circuit briefly described and lill-ustrated in Figure 5 has several advantages, some of the more important being vthat'l it tends to exhibit improved linearity and stability. It will be obvious that phase responsive aswell .as amplitude responsive detecting, correlating andl indicating means may be employed. In phase' responsive systems it may be further desirable to adjust the natural elastic period of one rod to be' slightly higher'and that of the other rod to be slightly lower ,than the period of the exciting frequency.

Figure 6 illustrates a regenerative means to vexcite longitudinal vibrations in a rod by the phenomenon of .magnetostriction and is well known in the art of magnetostriction generators. With reference to Figure 6, magnetostrictive rod 63 is firmly clamped at its nodal midpoint by supports 6| and 52. Windings 63 and 54 are grid and f plate windings for vacuum tube 55. Condenser 55 is chosen to resonate coil 64 at the' desired operating frequency, which is the resonant 'frequency of period of the rod. Pre-magnetization of rod 5l is accomplished by the steady plate current flow through coil 54 supplied by battery 61'. It is to be understood that rod 60 may likewise be coupled to a transducer and that subsequent circuits for detecting, correlating and. indicating amplitude, phase or waveform change may be employed. It is to be understood further that currents within the regenerative circuit will `bemodiiied by the direction and magnitude of the external magnetic field and that circuits for indicating these modlfcations may be used. Such circuits are well known in the electronic art and, for example, conveniently may be represented by meter M are not repeated for the sake of brevity. Y Figure 7 illustrates another regenerative means to excite longitudinal vibrations in a rod by the phenomenon of magnetostriction. With reference to Figure 7, magnetostrictive rod 58 is nodally clamped by supports 69 and 18. Battery 1I, rheostat 12 and coils 13 cooperate to pre-magnetize rod 68 to the desired value.` Piezoelectrlc transducer 14 is coupled to amplifier 15 and the amplified output is returned to rod 68 through r coils 15. In the manner well known to those versed in the art, the system oscillates at the natural elastic period of the mechanical feedback link. rod 68. It is to be understood that similar circuits for detecting amplitude, p hase and waveform changes occasioned by the magnitude or direction of an external magnetic field may be employed. The circuits illustrated ln Figures s and '1. that is regenerative driving circuits for the magnetostrictive element, have the advantage that resonance is always maintained between the natunickel rod will become shorter, the change in length being proportional to the intensity of the magnetic field and not upon the polarity of the the frequency of the exciting field. When a permanent magnetic field is superimposed upon the longitudinal axis of the rod, in addition to the alternating field, the rod will tend to vibrate at the frequency of the exciting field `rather than at twice the frequency.

For example, a rod approximately ten inches long has a fundamental elastic period of longitudinal vibration of approximately kilocyoles. When such a rod is not permanently or otherwise pre-magnetized and is introduced into a 10 kilocycle alternating magnetic field, it will tend to vibrate at twice its fundamental elastic period or `kilocycles. This corresponds to the second harmonic of the fundamental elastic period of the rod. As a permanent magnetic field is applied to the longitudinal axis oi' the rod, its 20 kilocycle output will diminish and its 10 kilocycle output will increase.

`Iligure 8 illustrates another modification of the present invention incorporating this principle. Magnetostrictive rod 11 may be supported by clamps 19 and resilient material 19. A method of mounting is preferred lwhich will permit both rst and second harmonic vibration. Exciting winding 99 is coupled through leads 8| and 92 to a source of alternating current which has a period closely matching the/fundamental elastic period 91, excited through resistor 99 and ground, 'are tuned to the second harmonic of the fundamental period of rod 11 and the output of.' this portion of the circuit is coupled through rectifier 98 to the remaining half of the bridge metering circuit comprising potentiometer 94 and filter condenser 99. Indicating meter |99 is connected across potentiometer 9| as shown in previous modifications. l

With the longitudinal axisof unmagnetized rod 11 perpendicular to the lines of force of an external magnetic field, the rod is set into resonant longitudinal vibration by an exciting voltage equal in frequency to the natural elastic period of the rod. Rod 11 vibrates at twice its fundamental elastic period, or the second harmonic. Transducer 83 transforms this mechanical motion into a voltage of equivalent periodicity, whichis ampliiied by amplifier 86 and passed through resistor 99, filter circuit 2f and rectier 98 to potentiometer 94 and condenser 99, unbalan'cing the bridge and causing meter |99 to deflect in one direction. As the longitudinal axis of unmagnetiaed rod 'l1 is caused to become more nearly parallel to the lines of force of an external magnetic field, the 10 lrilocycle` output of the rod increases andthe 2'0 kilocycle 'output decreases.

This change isv detected by transducer 93, amplifier 99 and lter circuit f, causing more voltage to ,appear across condenser 95 and less to appear `across condenser 99, further causing meter |99 to deflect in the reverse direction.

Figure 9 illustrates a'further modication of the present invention incorporating the foregoing principle, with rod |9| supported by clamps |92 and resilient material |93. A method 'of mounting is preferred` which will permit both fundamental and harmonic vibration. Exciting winding |9l is coupled through leads |95 and |99 to a source of alternatingcurrent having a period closely matching that of the natural elastic fundamental period of rod I9|. In this embodiment of the invention, rod |9i may be pre-magnetized lslightly as will be later explained. Such premagnetization may be permanent or electromagneticaily induced as discussed in connection with previous embodiments. Transducer |91,lmechan ically coupled to rod |9l, is further electrically connected through harmonic wave analyser |99 to harmonic content meter |99. Circuits for analysing the harmonic content of waveforms are well known in the electronic art and such instruments for indicating harmonic content directly are commercially available. Harmonic wave analyzer |98 and harmonic content meter |99 are found commercially in the model 30G-A Harmonic Wave Analyzer produced by the Hewlett- Packard Company of Palo Alto, California.

With slightly magnetized magnetostriction rod |9| excited by an alternating eld equal in frequency to the fundamental elastic period 'of rod |9l, transducer |91 will generate a voltage which will contain predominantly a sine Wave of the fundamental longitudinal `elastic period of the rod. When a steady external magnetic fleld is applied along the longitudinal axis of the rod such that it is bucking the pre-magnetized condition of the rod, the voltage output from 'transducer |91 will exhibit a wave shape containing the fundamental plus a second harmonic com- I ponent. The harmonic wave analyser will detect these differences and the meter of the wave analyzer circuit may be calibrated in terms of the direction and/or magnitude of the magnetic neld surrounding the rod.

The operation connected with Figure 9 can be explained best by reference again to Figure 3.'

axis. One half of the exciting field cycle will attempt to reduce H to zero and, in doing so, will move beyond zero, causing I-I to increase slightly. This phenomenon will introduce a second harmonic component in the Waveform from the transducer and this component may be detected by means similar to that disclosed or some equivalent thereof. By controlling the premagnetization, the appropriate centeroperating point on the curve in Figure 2 may be selected for desired operatingcharacteristics.

increase in the'fleld intensity H will cause the rod to lengthen land a further increase'in H will cause the rod to shorten. For convenience, as-

sume this point to be the "critical deflection" point. It will be apparent to those versed in the art that this point may be selected as the center operating point and that, as a change in H causes the center operating point to shift along the curve, there will follow a change in phase between the exciting and output voltages as the operating point passes through the zero deflection point. Further, because magnetization changes the dimensions of certain materials, it also shifts the natural elastic period of vibration of these materials. It' is obvious that means responsive to the change in natural period may be employed to detect and indicate the influence of the external field under observation. Such means may include a modified discriminatorcircuit familiar to those versed in the electronic art. i

It will be appreciated that, if desired, the magnetostrictive element may be supplied with an electro-mechanical or servo system responsive to its output to keep it oriented in any desired posi-A tion with regard to the externalfield. Such systems are well known in the arts of electronics and navigation. f

Further, it will be appreciated that certain benefits will accrue through the selection of mag- 4netostriction elements having particularly suitable elastic, magnetic and other physical properties. Still further, the system allows a wide choice as to operational frequency over the sonic and ultrasonic range. Equipment may be deusual lower frequency vibrations.

While I have shown and described in detail several preferred embodiments of my invention, I am aware that various changes and modifications may occur to those versed in the art without departing from the invention. Therefore, it is aimed in the appended claims to cover any and all such changes and modifications which fallwithin the true spirit and scope of the invention.

I claim:

1. In a magnetic field responsive device, a magneto-strictive element nodally supported for a preferred mode of substantially undamped vibration, coil means excited from a constant frequency source of periodically varying voltage and magnetically linked with said yelement to magnetostrictively vibrate said element, detector means coupled to said element sensible to vari` ances in the characteristics of magnetostrictlve vibrations of said element and indicator means f coupled to said detector -means arranged to indi- .40 signed which is not detrimetally' effected by the to variances in the characteristics of magneto- 12 to indicate variances in the characteristics of magnetostrictlve vibrations of said element.

3. In a magnetic neld responsive device, a mag- 4 netostrictive element nodally supported for a preferred mode of substantially undamped vibration, means independent of ambient magnetic ilclds to magnetize said element, coil means excited from a constant frequency source of periodically varying voltage and magnetically linked with said element to magnetostrictively vibrate said element, detector means coupled to said element sensible to variances in the characteristics of magnetostrictlve vibrations of said element and indicator means coupled to said detector means arranged to indicate variances in the characteristics ofmagnetostrictlve vibrations 'of said element,

4. In a magnetic field responsive device, a magnetized magnetostrictlve element, nodally supported for apreferred mode of substantially undamped vibration, coil means excited from a constant frequency source of periodically varying voltage and magnetically linked with said element to. magnetostrictively vibrate said element -and detector means, coupled to saidl element, arranged -t-o indi-cate variances in the amplitude of magnetostrictlve vibrationsof said element. V

5. In a magnetic field responsive device, a magnetized magnetostrictlve element nodally supported for a preferred mode of substantially undamped vibration, coil means excited from a constant frequency sourceof periodically varying voltage and magnetically linked with said element to. magnetostrictively vibrate said elei ment and detector means, coupled to said eleto indicate variances in' the waveform ofmagnetostrictive vibrations of said element.

7. In a magnetic field responsive device, a magnetostrictlve element nodally supported for a preferred mode of substantially undamped vibration, coil means excited from a constant frequency source of voltage periodically varying at a frequency attuned to a natural period of vibraltion of said element and 'magnetically linked with said element to magnetostrictively vibrate said element and detector means, coupled to said element, arranged to indicate variances in the characteristics of magnetostrictlve vibrations of said element'.

8. In a magnetic field responsive device, a magnetostrictlve element vnodally supported for a preferred modeof substantiallyundamped vibration, coil and circuit means magnetically linking said element arranged to include said element as a magneto-mechanical feedback path in a regenerative electrical circuit, regenerative elec- -trical circuit means coupled to said coil means and means, coupled to said circuit means, arranged to indicate variances in the characteristics of magnetostrictlve vibrations of sai-d element.

9. In an earth inductor compass, a magnetostrictlve element nodally clamped for a preferred' mode of substantially undamped vibration, means supporting said clamping means for -angular movement of said element relative to the earths magnetic field, means independent of the earths magnetic eld to magnetize said element, coil means excited from a constant frequency source of periodically varying voltage and magnetically linked with said element to magnetostrictively vibrate said element and detector means, coupled to said element, arranged to indicate variances in the characteristics of magnetostrictive vibrations of said element as influenced by the earths magnetic iield, whereby the direction of .-the earths magnetic meridian may be determined.

10. In a magnetic field responsive device, a

magnetostrictive element, nodal clamping means aranged to support said element for lsubstantially undamped vibrations and for angular movement relative to an ambient magnetic field, means independent of said ambient magnetic eld to on said element a steady magnetic field of predetermined magnitude and direction, superimposing onsaid element an alternating-magnetic fleldof predetermined magnitude, direction and frequency and detecting and indicating variances in the characteristics of magnetostrictive vibrations of said element as influenced by the magnetic iield to be detected and indicated.

\ magnetic field, detecting and indicating the charmagnetize said element, coil means excited from a constant frequency source of periodically varying voltage and magnetically linked with said element to magnetostrictively vibrate said element and detector means, coupled to said eleyment, arranged to indicate variances in the characteristics of magnetostrictive vibrationsI of said element as influenced by the direction of said ambient magnetic iield.

11. The method of detecting and indicating the magnitude and direction of a magnetic eld comprising, subjecting a magnetostrictive element simultaneously to a first magnetic field to be detected and a second alternating magnetic eld of predetermined magnitude, direction and frequency and detecting and indicating the variance in the characteristics of magnetostrictive vinbrations of said magnetostrictive element as influenced by the magnitude and direction of the magnetic field to be detected and indicated.

12. The method of detecting and indicating the magnitude and direction of a magnetic field comprising, subjecting a magnetostrictive element simultaneously to a first magnetic neld to `ne detected. a second steady magnetic field of predetermined magnitude and direction and a third alternating magnetic field of predetermined ment as influenced by the magnitude'and direc-` detected and 4 tion o1' the magnetic field to be indicated. f

14. The method of detecting and indicating the magnitude and direction of a magnetic field comprising, immerslng a magnetostrictive eleacteristics of magnetostrictive vibrations of said element, varying the magnitude of the field to be detected and simultaneously detecting and indicating the variance in the characteristics of magnetostrictive vibrations of said element.

I6. The method of detecting and indicating the Vdirection of a magnetic iield comprising, immersing a magnetostrictive element in the field to be detected, magnetically biasing said element with a-second magnetic field, magnetostrictively vibrating said element with a third periodically varying magnetic iield, detecting and indicating the characteristics of magnetostrictive vibrations of said element, varying the direction of the field to be detected with respect to said element and detecting and indicating the variance in the characteristics of magnetostrictive vibrations of said magnetostrictive element.

17. 'Ihe method of detecting and indicating the magnitude and direction of a magnetic eld comprising; immersing a magnetostrictive element in the field to be detected, magnetostrictively vibrating said magnetostrictive element, detecting the characteristics of magnetostrictive vibrations of said magnetostrictive element, varying the position of said magnetostrictive element with respect to the magnetic field to be detected and indicated, and simultaneously detecting and indicating the variance in the characteristics of magnetostrictive vibrations of said magnetostricment in the nele'. to be detected, superimposing tive element as iniiuenced by the magnitude and directionof the indicated.

l ROBERT G. aowE.

REFERENCES (/lITED 'I'he following referenices are of record in the Riggs Feb. 27, .1945

magnetic eld to be detected and 

